The present invention relates to the field of optical detectors for converting light into an electrical signal.
Schottky diodes are semiconductor structures replacing the current blocking p-n junction with a metal-semiconductor junction. Such diodes can out-perform standard p-i-n diodes because of their lower capacitance. A special case of the Schottky diode detector is the lateral Metal-Semiconductor-Metal (MSM) detector. Using the same metal for the anode and cathode makes the device easier to fabricate. MSM detectors are currently fabricated in a planar geometry as compared to the vertical geometry of the p-i-n diode. As with all optical detectors, the speed of the MSM can be dictated by carrier life time, carrier transit time, or parasitic RC effects. In the past, decreasing the carrier lifetime by addition of defects or recombination centers resulted in fast pulse response, but seriously degraded sensitivity. RC effects are reduced by a planar interdigitated layout. More recently transit time limited devices have been fabricated with sub-micrometer electrode finger spacing. These small linewidths are difficult to fabricate. Modeling of sub-micrometer finger spacing MSM devices have revealed several interesting aspects of device design. The electric-field lines becomes localized at the surface of the device. In this case, carriers generated in the neutral region far from the surface must diffuse into the high field region before collection, limiting the ultimate speed of these devices.
Fabrication of MSM detectors requires the formation of high quality Schottky barriers, with low reverse leakage current. This is not easily achieved on a low band-gap material, such as InGaAs. However, a barrier enhancing layer, such as InAlAs, improves the performance of such devices.